For a transport system with an optical submarine cable, there has been a request to use a redundant system optical transport system as the maintenance functions of optical transport devices in preparation for a failure occurring in a submarine cable. An outline diagram of the redundant system optical transport system is shown in FIG. 10.
As shown in FIG. 10, a client transport device 101 that is network protection equipment (NPE) placed in a first land section has an output port connected to an optical coupler 102. The optical coupler 102 has an output port connected to a transponder 103, and another output port connected to an optical switch 106. Further, the optical switch 106 has input ports respectively connected to the optical coupler 12 and an optical switch 208, and selects one of them as an input source according to a control signal from a monitoring control unit (not shown). Further, the optical switch 106 has an output port connected to a redundant transponder 107. The above-mentioned optical coupler 102, the above-mentioned transponder 103, the above-mentioned optical switch 106, the above-mentioned optical switch 208, the above-mentioned redundant transponder 107, the above-mentioned monitoring control unit, and a transponder 204 and an optical coupler 205, for which an explanation will be omitted hereafter, are disposed in a land terminal station on a first land section side of a submarine section.
Then, when the redundant system optical transport system is in a normal operation state, a client signal sent out from the client transport device 101 is inputted, via the optical coupler 102, to the transponder 103. Outputted light from the transponder 103 is inputted to a transponder 104, which is opposite to the transponder 103, via an optical submarine cable. After that, the outputted light from the transponder 104 is inputted to a client transport device 201 placed in a second land section via an optical coupler 105. The above-mentioned transponder 104, the above-mentioned optical coupler 105, a redundant transponder 207 and an optical switch 206, which will be described below, an optical coupler 202 for which an explanation will be omitted hereafter, and a monitoring control unit (not shown) are disposed in a land terminal station on a second land section side of the submarine section.
Further, the optical switch 106 selects the optical switch 208 as the input source thereof, and the optical switch 208 selects the optical switch 106 as the output destination thereof. The optical switch 206 selects the optical switch 108 as the input source thereof, and the optical switch 108 selects the optical switch 206 as the output destination thereof. As a result, the four optical switches 106, 108, 206, and 208 and the two redundant transponders 107 and 207 are configured to have a form in which a signal (OTU frame) loops through them.
The same goes for a path extending from the client transport device 201 to the client transport device 101, and the explanation of the path will omitted hereafter.
On the other hand, when a failure occurs in the optical submarine cable, the monitoring control unit detects this failure and controls the optical switch 106 to select the optical coupler 102 as the input source thereof. The monitoring control unit further changes the output destination of the optical switch 108 to the optical coupler 105. As a result, the outputted light (client signal) from the optical coupler 102 is outputted to the redundant transponder 107 via the optical switch 106. Then, the outputted light from the redundant transponder 107 is inputted to the redundant transponder 207, which is opposite to the redundant transponder 107, via the optical submarine cable. The outputted light from the redundant transponder 207 is then inputted to the client transport device 201 via the optical switch 108 and the optical coupler 105.
The same goes for the path extending from the client transport device 201 to the client transport device 101 at the time of the occurrence of the failure, and the explanation of the path will omitted hereafter.
The ITU-T recommendation G.709 standard “Interfaces for the Optical Transport Network (OTN)” is standardized as the maintenance service switching protocol associated with the switching control on optical switches at the time of the occurrence of a failure, and is used in an optical transport system (for example, refer to nonpatent reference 1).
The OTU overhead mapping defined in the ITU-T recommendation G.709 standard is shown in FIG. 11. In the OTU overhead mapping shown in FIG. 11, an OPUk payload is a location where transmit information is stored, and an overhead is a location which is attached to the head of the payload and where transport information and a transport path alarm are stored. According to the ITU-T recommendation G.709 standard, APS/PCC (Automatic Protection Switching Coordination channel/Protection Communication Control channel) bytes which are a switching overhead area are used for monitoring control.